This invention relates to electronic oscillators and, in particular, to a source-coupled oscillator capable of operating over an exceptionally wide frequency range with a minimal change in amplitude.
FIG. 1 is a circuit diagram of a known source-coupled oscillator 10, which uses AC source-coupled differential-current switches (MESFETs X1, X2) to steer current to loads L1, L2. The voltages at loads L1, L2 are buffered by source-follower MESFETs X3, X4, level shifted by level shift devices Z1, Z2, and cross-coupled to the gates of the opposite ones of MESFETs X1, X2, respectively. Constant current source S5 provides a current for source-follower X3 and level shift device Z1, and constant current source S6 provides a current for source-follower X4 and level shift device Z2. Level shift devices Z1 and Z2 can be diodes, batteries or any other component capable of inserting a constant voltage drop into the circuit.
The frequency of oscillator 10 is proportional to the size of the currents I1, I2 supplied by current sources S1, S2, respectively, and inversely proportional to the capacitance of capacitor C1. The period of the oscillations (T) can be expressed by the relationship:
Txcx9c(V/I)xc3x97C1
where V is the signal swing ((I1+I2)xc3x97Lx), I is the magnitude of the currents I1, I2, and C1 is value of capacitor C1. This is a capacitive slew rate relationship regulating the discharge time of capacitor C1 by the currents I1, I2. Since, in this example, I1 is equal to I2, the duty cycle of the oscillations is 50%. If I1 is not equal to I2, then the duty cycle of the oscillation will be equal to I1/I2.
Oscillator 10 is essentially controlled by positive feedback paths which run: (a) from node 1 at the terminal of load L1, through source-follower MESFET X3 and level shift device Z1 to the gate of MESFET X2; and then from node 2 at the terminal of load L2, through MESFET X4 and level shift device Z2 to the gate of MESFET X1; and (b) from node 2 at the terminal of load L2, through source-follower MESFET X4 and level shift device Z2 to the gate of MESFET X1, and then from node 1 through source-follower MESFET X4 and level shift device Z1 to the gate of MESFET X2. The net result is that an increase in the current through MESFET X1 tends to turn MESFET X1 further on, and an increase in the current through MESFET X2 tends to turn MESFET X2 further on.
When oscillator 10 is initially turned on, noise fluctuations cause the currents through MESFETs X1 and X2 to vary (i.e., because of noise, it is impossible for the currents through MESFETs X1 and X2 to remain perfectly constant). Assume that initially the current through MESFET X1 is increasing. With the current through MESFET X1 and load L1 is increasing, the voltage at node 1 falls. This falling voltage is coupled through source-follower MESFET X3 and through level shift device Z1 to the gate of MESFET X2. Since in this embodiment MESFETs X1 and X2 are N-channel devices, the effect of lowering the voltage at the gate of MESFET X2 is to reduce the source-to-gate voltage Vgs of MESFET X2, thereby reducing the current through MESFET X2. As the current through MESFET X2 becomes smaller, the voltage at node 2 rises. This increasing voltage is coupled through source-follower MESFET X4 and level shift device Z2 to the gate of MESFET X1. This increases Vgs in MESFET X1 and further increases the size of the current through MESFET X1.
Capacitor C1 transmits the voltage at the source of MESFET X1 to the source of MESFET X2, following the normal capacitive lag. As MESFET X1 turns on, the voltage at its source rises, biasing the left side of capacitor C1 positively. This rising voltage is transmitted, to the source of MESFET X2 and reinforces the reduction of Vgs in MESFET X2. The right side of capacitor C1 is biased negatively.
This process continues until MESFET X1 is fully turned on and MESFET X2 is fully turned off. The circuit is now halfway through one cycle of oscillation. With MESFET X2 off, current source S2 draws current from the right side of capacitor C1. This starts to pull the voltage at the source of MESFET X2 down, increasing Vgs in MESFET X2 and beginning to turn MESFET X2 on. As the current through MESFET X2 increases, the voltage at node 2 begins to fall, and this falling voltage is transmitted through source-follower MESFET X4 and level shift device Z2 to the gate of MESFET X1. This reduces the Vgs of MESFET X1 and begins to turn MESFET X1 off. As the current through MESFET X1 decreases, the voltage at node 1 rises. This rising voltage is transmitted through source-follower X3 and level shift device Z1 to the gate of MESFET X2, further increasing the Vgs of MESFET X2. At this point in the cycle, the increasing current through MESFET X2 begins to charge the right side of capacitor C1. This increasing voltage is transmitted to the left side of capacitor C1, further reducing the Vgs of MESFET X1. The process continues until MESFET X1 is fully turned off and MESFET X2 is fully turned on, completing one full cycle of oscillation. The current I1 then starts to discharge the left side of capacitor C1, and the cycle is repeated.
In this conventional oscillator, the frequency is set by adjusting the magnitude of the currents I1 and I2 and/or the size of capacitor C1. The frequency increases as I1 and I2 increase and decreases as I1 and I2 decrease. A problem with this arrangement, however, is that as the size of I1 and I2 varies, the amplitude of the output signal also varies. For example, the signal swing at node 1 is directly related to the magnitude of (I1+I2)xc3x97L1), and the signal at node 1 is translated through source-follower MESFET X3 and level shift device Z1 to become the OUT1 signal. Likewise, the signal swing at node 2 directly related to the magnitude of (I2+I1)xc3x97L2, and the signal at node 2 is translated through source-follower MESFET X4 and level shift device Z2 to become the OUT2 signal. Thus increasing (or decreasing) the frequency of oscillator 10 has the undesirable side effect of also increasing (or decreasing) the amplitude of the output signal. Increasing amplitude means increasing output slew times, and decreasing amplitude means decreasing output slew times. Therefore, as the current is increased to increase the frequency of the oscillator (or decreased to decrease the frequency of the oscillator), the change in the voltage swing that the circuit must slew through is acting in opposition to the desired change in frequency. This limits the oscillator to a much narrower tuning range than would be assumed from the relationship Txcx9c(V/I)xc3x97C1. The gain issue becomes a problem as the current is reduced to a level below that at which the source-coupled amplifier""s gain drops below one. The problem of varying amplitude also creates issues with available bias and supply constraints.
There is, accordingly, a real need for a source-coupled oscillator that is able to operate at a substantially fixed amplitude over a wider frequency range than a conventional oscillator of the kind described above.
In accordance with this invention, a source-coupled oscillator includes a first MESFET connected with a first load in a first current path and a second MESFET connected with a second load in a second current path. The first MESFET is supplied with a constant current (I1) and the second MESFET is supplied with a constant current (I2). A capacitor is connected between a source of the first MESFET and a source of the second MESFET. A first feedback path is connected between the first load and a gate of the second MESFET; and a second feedback path is connected between the second load and a gate of the first MESFET.
The oscillator also two pairs MESFETs, each of the differential pairs being supplied with a constant current. In the first differential pair, one of the MESFETs is connected to switch a current into the firs t current path and ha s a gate that is coupled to the second feedback path; the second MESFET in the first differential pair is connected to switch a current into the second current path and has a gate connected to the first current path. In the second differential pair, one of the MESFETs is connected to switch a current into the first current path and has a gate that is coupled to the second current path; the second MESFET in the second differential pair is connected to switch a current into the second current path and has a gate connected to the first feedback path. The first differential pair is supplied with a constant current (I3), an d the second differential pair is supplied with a constant current (I4).
The frequency of the oscillator can be adjusted by changing the size of the currents I1 and I2, which flow through the first and second MESFETs. To maintain the amplitude of the output signal constant, the constant currents I3 and I4 are adjusted such that the sum of I2, I2, I3 and I4 remains the same. I3 and I4 act as compensating currents to ensure that th e total current flowing through the loads remains constant.
Using the source-coupled oscillator of this invention, the frequency c an b e changed over very broad ranges while maintaining the amplitude of the output signal substantially constant.